I issue my SMTP send requests in a controlled loop, and only attempt to send 2 a second. While undeliverables may build in my queue, almost all good addresses get sent as soon as my program asks them to.

This is for a couple of reasons.

I don't want to trigger spam filters. We only send to our inhouse list (real customers), but that doesn't mean if I attempt to send a bunch to the same ISP in a short while they won't be flagged as spam.

Also, this is a shared corporate internet connection. It is not that fast, something like 384KB. If I really let the mail server rip, I'd swamp the corporate traffic.